+    // Transitions for 3state checkbox must be done manually, GTK's checkbox
+    // is 2state with additional "undetermined state" flag which isn't
+    // changed automatically:
+    if (cb->Is3State())
+    {
+        GtkToggleButton *toggle = GTK_TOGGLE_BUTTON(widget);
+
+        if (cb->Is3rdStateAllowedForUser())
+        {
+            // The 3 states cycle like this when clicked:
+            // checked -> undetermined -> unchecked -> checked -> ...
+            bool active = gtk_toggle_button_get_active(toggle);
+            bool inconsistent = gtk_toggle_button_get_inconsistent(toggle);
+
+            cb->GTKDisableEvents();
+
+            if (!active && !inconsistent)
+            {
+                // checked -> undetermined
+                gtk_toggle_button_set_active(toggle, true);
+                gtk_toggle_button_set_inconsistent(toggle, true);
+            }
+            else if (!active && inconsistent)
+            {
+                // undetermined -> unchecked
+                gtk_toggle_button_set_inconsistent(toggle, false);
+            }
+            else if (active && !inconsistent)
+            {
+                // unchecked -> checked
+                // nothing to do
+            }
+            else
+            {
+                wxFAIL_MSG(wxT("3state wxCheckBox in unexpected state!"));
+            }
+
+            cb->GTKEnableEvents();
+        }
+        else
+        {
+            // user's action unsets undetermined state:
+            gtk_toggle_button_set_inconsistent(toggle, false);
+        }
+    }
+